Battlefield bad company 2 Vietnam dlc serial code problem

Hi, I just purchased Bad Company and the Vietnam DLC from steam. I got in fine and my Battlefield Bad Company code worked fine, but whilst stupidly fumbling around trying to find the Vietnam section, I redeemed my Vietnam code in the box at the top saying, 'redeem product code'.

So now I can't play Vietnam because it tells me someone else is already using it. I also can't buy a new code from steam because they say I already own one.

If someone can help me with this I would be overjoyed, as I played this game for hours on the consoles!